1. Field of the Invention
The present invention relates to the field of optical scientific instrumentation. More specifically, the present invention relates to an all-graphite bearing assembly as part of a Michelson interferometer.
2. Discussion of the Related Art
An optical interferometer used in a scientific analytical instrument relies on the interference of superimposed optical beams as part of the interrogation means. When configured as a Michelson Fourier-Transformed infrared (FTIR) instrument, the optical output of the interferometer is called an interferogram. The FTIR interferometer itself often includes a beam splitter and two mirrors, one that is conventionally stationary, and one which is conventionally mobile. The mobile mirror moves along the optic axis while staying optically perpendicular to the light beam at all times. The movement of the mobile mirror is often desired to be feedback-controlled in order to hold the mirror velocity constant so that the analytical radiation that passes through the interferometer produces an accurate interferogram. Moreover, the mobile mirror is also often part of an assembly that includes either air bearings or mechanical pivot-type bearings which require costly close tolerance machining and assembly for controlling the tilt of the movable mirror as it moves. Air bearings offer higher performance but are expensive and require an air compressor and filter to supply compressed air. Mechanical pivot-type bearings can have errors in the mirror alignment as the mirror moves. Such alignment problems worsen at long stroke lengths, thus limiting the stroke length and system resolution. Moreover, these types of bearings are subject to wear and degradation. In addition, mechanical bearings also have poor damping and tend to capture or generate mechanical and acoustical vibrations, thereby causing noise in the system output data.
To somewhat address the aforementioned conventional problems, a mirror assembly was manufactured that utilized graphite/glass combinations as part of the construction. Such an assembly is described and claimed in, U.S. Pat. No. 5,896,197, entitled, “INTERFEROMETER HAVING A GLASS GRAPHITE BEARING” issued Apr. 20, 1999, to John M. Coffin, the disclosure of which is incorporated by reference in its entirety, including the following: “[a] bearing for allowing the movement of a movable mirror in a Michelson interferometer includes a stationary hollow glass cylinder and a movable assembly which includes the movable mirror and at least one graphite member, the graphite member being slidably disposed within the bore of the glass cylinder.” Part of the basis for such an assembly was to beneficially improve vibration damping because of predictable friction between the different parts and also reduce costs because it enabled low power control systems due to the lightweight nature of the configuration. However, such assemblies have inherent problems in manufacturing and reliability. For example, current manufacturing processes for such glass/graphite configurations calls for hand fitting of the glass tube to the piston, which often leads to large variances between batches of parts. Another problem encountered with such assemblies is that surface imperfections in the glass tube often result in tilt and drive jitter during operation of the final assembly in the interferometer. Finally, buildup of friction of the graphite piston, as evidenced by graphite dust in the glass tube, resulted in the need for higher drive voltages to be applied and such buildup eventually causes system downtime based on the need to clean the glass tube and graphite piston periodically.
Accordingly, the present invention addresses the need for an improved moving mirror assembly system as utilized in scientific optical interferometers, such as, a Fourier Transform infrared (FTIR) interferometer. In particular, such a novel design, as disclosed herein, provides for an improvement of the design cited in U.S. Pat. No. 5,896,197, to include but of which is not limited to: manufacturing parts of the assembly with tolerances and surface finishes that reduces friction and stiction of coupled parts, enabling tighter tolerances so as to improve system performance such as reduced, jitter and further reduction of the weight of the overall assembly which provides for low power control systems to be utilized for actuating the movable mirror while also enabling higher scanning speeds.